Ion association behaviors in the initial stage of calcium carbonate formation: An ab initio study

In this work, we performed static density functional theory calculations and ab initio metadynamics simulations to systematically investigate the association mechanisms and dynamic structures of four kinds of ion pairs that could be formed before the nucleation of CaCO3. For Ca2+-HCO3- and Ca2+-CO32- pairs, the arrangement of ligands around Ca2+ evolves between the six-coordinated octahedral structure and the seven-coordinated pentagonal bipyramidal structure. The formation of ion pairs follows an associative ligand substitution mechanism. Compared with HCO3-, CO32- exhibits a stronger affinity to Ca2+, leading to the formation of a more stable precursor phase in the prenucleation stage, which promotes the subsequent CaCO3 nucleation. In alkaline environments, excessive OH- ions decrease the coordination preference of Ca2+. In this case, the formation of Ca(OH)+-CO32- and Ca(OH)2-CO32- pairs favors the dissociative ligand substitution mechanism. The inhibiting effects of OH- ion on the CaCO3 association can be interpreted from two aspects, i.e., (1) OH- neutralizes positive charges on Ca2+, decreases the electrostatic interactions between Ca2+ and CO32-, and thus hinders the formation of the CaCO3 monomer, and (2) OH- decreases the capacity of Ca2+ for accommodating O, making it easier to separate Ca2+ and CO32- ions. Our findings on the ion association behaviors in the initial stage of CaCO3 formation not only help scientists evaluate the impact of ocean acidification on biomineralization but also provide theoretical support for the discovery and development of more effective approaches to manage undesirable scaling issues.

Multi-step nucleation pathway of C-S-H during cement hydration from atomistic simulations

The Calcium Silicate Hydrate (C-S-H) nucleation is a crucial step during cement hydration and determines to a great extent the rheology, microstructure, and properties of the cement paste. Recent evidence indicates that the C-S-H nucleation involves at least two steps, yet the underlying atomic scale mechanism, the nature of the primary particles and their stability, or how they merge/aggregate to form larger structures is unknown. In this work, we use atomistic simulation methods, specifically DFT, evolutionary algorithms (EA), and Molecular Dynamics (MD), to investigate the structure and formation of C-S-H primary particles (PPs) from the ions in solution, and then discuss a possible formation pathway for the C-S-H nucleation. Our simulations indicate that even for small sizes the most stable clusters encode C-S-H structural motifs, and we identified a C4S4H2 cluster candidate to be the C-S-H basic building block. We suggest a formation path in which small clusters formed by silicate dimers merge into large elongated aggregates. Upon dehydration, the C-S-H basic building blocks can be formed within the aggregates, and eventually crystallize.

Recovery of phosphate as hydroxyapatite by fluidized bed homogeneous crystallization technique

Phosphorous recovery from aqueous solutions gained substantial attention and this not only secure the food demand but also curtail the pollution of freshwater courses. In the current study, authors employed novel fluidized bed homogeneous crystallization (FBHC) technique to granulate the phosphorous as hydroxyapatite (HAP). FBHC technique nurtures the formation of high pure HAP crystals without seed addition and potential technique to recover phosphorous compared to other techniques. The key operational parameters influencing the HAP crystallization were analyzed prior to FBHC by batch analysis. From the batch study results, the range of pH and calcium to phosphorous molar ratio fixed for FBHC studies. Maximum phosphate removal and granulation efficiencies obtained were 91.25% and 82.55%, respectively, at 500 mg/L phosphate concentration, pH 12, and calcium to phosphorous molar ratio 1.65. Box-Behnken design of response surface methodology was employed for evaluating interaction impact of process parameters on granulation efficiency. Granulation efficiency of 79.74% was attained at pH 11.83, calcium to phosphorous molar ratio 1.637, and reaction time 70.73 h.

Effect of gamma-irradiation on complexation of humic substances with divalent calcium ion

The effect of gamma-irradiation doses of 0, 10, 100, and 500 kGy at the dose rates of 1 or 0.1 kGy/h on the molecular and chemical properties of humic substances (HS) were investigated using total organic carbon measurements, UV-Vis spectrometry, ¹³C nuclear magnetic resonance, and acid-base titration. A possible mechanism of the radiolysis on HS was also speculated. The complexation ability of irradiated HS with Ca²⁺ ions was studied using a Ca ion-selective electrode. The apparent formation constants of the Ca-HS complexes increased as the irradiation dose increased, and this was attributed to the relative increase in the ratio of phenolic -OH to carboxylic groups of HS. The contribution of the phenolic -OH groups to Ca-HS complexes was suppressed at pH 5 owing to its high acid dissociation constants. In addition, the radiation dose rates of 1 and 0.1 kGy/h did not significantly affect the properties of HS and the apparent formation constants of the Ca-HS complexes.

Biomimetic synthesis of calcium carbonate under phenylalanine: Control of polymorph and morphology

In biomineralization, organisms have the abilities to produce biominerals with superior properties. One of the most attractive features of biominerals is the presence of the proteins consisting of different contents of amino acids in crystals. In the present work, L-phenylalanine (Phe) was used as an additive for the controllable crystallization of calcium carbonate (CaCO₃). The obtained CaCO₃ crystals were characterized by field emission scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), elemental analysis and high-resolution transmission electron microscopy (HRTEM). The experimental results suggest that single calcite crystals are formed at low Phe concentrations. High concentrations of Phe inhibit the nucleation and growth of calcite, and promote the formation of vaterite crystals with solid or hollow structures. The morphology and crystal form of CaCO₃ are also significantly affected by the flow rate of CO₂. After that, a possible mechanism (competition mechanism) action of Phe in the formation of CaCO₃ is proposed. Finally, the effects of temperature on the formation of vaterite were determined to explore the growth mechanism of hexagonal vaterite. The work of controlling the preparation of CaCO₃ crystals in the presence of Phe will help us to imitate and learn nature, and bring new insights into understanding bionics. Meanwhile, it provides a new method for the synthesis of CaCO₃ biomaterials with different crystal forms and morphologies.

Chemo-Mechanical Model for the Expansion of Concrete Due to Alkali Silica Reaction

A chemo-damage model is proposed to predict the expansion caused by the alkali silica reaction (ASR). The model covers the formation of the pre-expansion gel driven by alkali and the swelling of the gel driven by water. The swelling capacity of the ASR gel is quantified by the sodium to calcium ratio in the pore solution. The bound alkali in the gel recycled by calcium is also considered in this model. Both external alkali supply and internal alkali released from aggregates are included. Several sets of experimental data are compared with the simulation results for the verification of the model.

Magnesium(II) d-Gluconate Complexes Relevant to Radioactive Waste Disposals: Metal-Ion-Induced Ligand Deprotonation or Ligand-Promoted Metal-Ion Hydrolysis?

The complexation equilibria between Mg2+ and d-gluconate (Gluc-) ions are of particular importance in modeling the chemical speciation in low- and intermediate-level radioactive waste repositories. NMR measurements and potentiometric titrations conducted at 25 °C and 4 M ionic strength revealed the formation of the MgGluc+, MgGlucOH0, MgGluc(OH)2-, and Mg3Gluc2(OH)40 complexes. The trinuclear species provides indirect evidence for the existence of multinuclear magnesium(II) hydroxido complexes, whose formation was proposed earlier but has not been confirmed yet. Additionally, speciation calculations demonstrated that MgCl2 can markedly decrease the solubility of thorium(IV) at low ligand concentrations. Regarding the structure of MgGluc+, both IR spectra and density functional theory (DFT) calculations indicate the monodentate coordination of Gluc-. By the potentiometric data, the acidity of the water molecules is higher in the MgGluc+ and MgGlucOH0 species than in the Mg(H2O)62+ aqua ion. On the basis of DFT calculations, this ligand-promoted hydrolysis is caused by strong hydrogen bonds forming between Gluc- and Mg(H2O)62+. Conversely, metal-ion-induced ligand deprotonation takes place in the case of calcium(II) complexes, giving rise to salient variations on the NMR spectra in a strongly alkaline medium.

Hydrotalcite and Hydrocalumite in Mortar Binders from the Medieval Castle of Portilla (Álava, North Spain): Accurate Mineralogical Control to Achieve More Reliable Chronological Ages

Mortars from different stratigraphic units at Portilla Castle (Alava, North Spain) have been analyzed for mineralogical characterization before radiocarbon dating. The mortar binder at Portilla Castle is composed not only of neoformation calcite but also of double-layered hydroxide (LDH) minerals such as hydrotalcite and hydrocalumite. The mineralogy of several fractions of the binder has been analyzed to determine the granulometric distribution of minerals in the binder. The continuous monitoring of mineralogy during the extraction of different grain size fractions has been performed by using a scanning electron microscopy (SEM), X-ray diffraction (XRD), and thermogravimetric analyses (TGA). Hydrotalcite and hydrocalumite-bearing mortar binders give older ages than expected since they introduce dead carbon into the system.

Comparison of the Ca2+ complexing properties of isosaccharinate and gluconate – is gluconate a reliable structural and functional model of isosaccharinate?

During the interactions of α-d-isosaccharinate and d-gluconate with Ca²⁺ in aqueous solution, differences rather than similarities prevail.

Calcium l-tartrate complex formation in neutral and in hyperalkaline aqueous solutions

In hyper-alkaline aqueous solutions, Ca²⁺ and l-tartrate (Tar²⁻) ions form CaTarH₋₁⁻_(aq) and CaTarH₋₂²⁻_(aq) complexes containing deprotonated alcoholate group(s).